The purpose of this Mentored Research Scientist Development Award (K01) is threefold. This award will allow me to: 1) learn three new skill sets: quantitative and mixed methodologies; multilevel modeling; and the integration of qualitative data and advanced quantitative modeling data; 2) broaden my understanding of substance use and HIV among substance users in a new cultural and environmental context (the US/Mexico border); and 3) facilitate my transition to becoming a productive NIH-funded independent investigator at the University of California San Diego. The training aims will be accomplished through a combination of specific workshops and coursework, a hands-on research project, and one-on-one mentoring with a Training Committee comprised of experts in the areas of HIV prevention research among vulnerable substance using populations, qualitative and mixed methods research, multilevel modeling, ethics, and drug use epidemiology research in international contexts (and specifically the US/Mexico border region). The research aims will be accomplished by conducting a mixed-methods study of social and structural aspects of sex work environments influencing HIV risk among male clients of FSW in Tijuana, Mexico. The research will be conducted as an independent adjunct to an existing NIH/NIDA-funded R01, DA029008 (Safer Sex Intervention for Male Clients of Female Sex Workers in Tijuana, Mexico aka Hombre Seguro, PI: Thomas Patterson), in order to capitalize on the infrastructure and expertise represented by that project. The US/Mexico border is experiencing a burgeoning HIV epidemic, concentrated among high-risk groups such as FSW, male clients, and drug users. While researchers have long applied mixed methods to HIV/STI prevention, few researchers have integrated rich information gleaned from qualitative methods with advanced statistical methods like multilevel, or hierarchical linear modeling. Further, researchers have yet to utilize mixed methods to provide a nuanced, contextual understanding of which sub-groups do and do not benefit from risk reduction interventions, as well as why and how. The proposed research will allow me to apply newly acquired skills in qualitative and mixed methods and integrating qualitative and multilevel modeling data to explore risk environments factors surrounding substance use in the context of sex with FSW in sex work venues. This work will be conducted in a particularly high-risk population (in Tijuana, HIV prevalence among female sex workers is 6% and 5% for male clients). Findings from the proposed research will be critical for developing multi-level interventions for substance using populations at risk for HIV in border regions and other international settings, and for the development of future mixed methods research integrating advanced quantitative techniques with qualitative approaches in HIV prevention. Further, developing skills in qualitative and mixed methods, and multilevel modeling will uniquely position me as one of only a handful of mixed-methods researchers in the field of substance use and sex work possessing such skills, and the only one in the Division of Global Public Health and the University of California San Diego.